The present invention relates to water-soluble, sulfoalkyl-containing, hydrophobically modified cellulose ethers, to processes for preparing them and to their use as protective colloids in polymerizations.
The preparation of vinyl polymers by free-radical polymerization in an aqueous, solvent-free medium makes it necessary to emulsify the hydrophobic monomers and, after polymerization is complete, to stabilize the polymer. For this reason, the polymerization of monomer systems comprising water-insoluble vinyl monomers in aqueous systems requires not only surfactants but also protective colloids which, on the one hand, have hydrophilic character and, on the other hand, should also have a dispersing action.
The quality of a polymer dispersion is decisively influenced by the choice of the protective colloid. Important quality criteria which can be influenced by the protective colloids are, for example, the stability, viscosity, rheology, the particle size of the polymer particles of the dispersion, and also the amount of coagulum which remains on filtering the dispersion through a sieve. The molecular weight is also influenced by the protective colloid. A further quality criterion is the water absorption of a film which has been produced by spreading and drying a dispersion. This property too is influenced by the protective colloid. In suspension polymerization, the protective colloid controls the particle size of the polymer formed.
It has been known for a long time that polymeric carbohydrates such as starch, dextrans and water-soluble cellulose derivatives are suitable protective colloids for water-based polymerization systems. The protective colloid used most frequently in the commercial production of polyvinyl acetate dispersions is hydroxyethylcellulose (Cellulose and its Derivatives, chapter 26, Ellis Horwood Limited 1985), which is produced on an industrial scale from cellulose and ethylene oxide.
The decisive process in the use of protective colloids in emulsion polymerization is regarded as being free-radical formation on the protective colloid and subsequent grafting of the monomer onto the colloid. The degree of grafting depends on the choice of free-radical initiator. Free-radical initiators customarily used are diazo compounds, redox initiators, organic and inorganic peroxo compounds. On the other hand, the degree of grafting also depends on the nature of the protective colloid. If the degree of grafting is low, the chosen concentration of the protective colloid must be appropriately high in order to achieve a sufficient effect. However, a high protective colloid concentration is undesirable, on the one hand, for cost reasons and, on the other hand, it also leads to increased hydrophilicity of the filmed polymer, in conjunction with increased water absorption.